Transmission system



Maly 12, 1942.

R. H. HERRlcK TRANSMISSION SYSTEM Filed Aug. 14, 1959 A'rroRNEYsPatented May 12, 1942'.

TRANSMISSION srs'rEM v Boswell H. Herrick, Oak Park,`lll., assigner toAssocia'tcd'Electrlc laboratories; Inc., lll., a corporation of vChicago,

Application August .14, 1939, serial No. 290,081

(ci. 11s-17o) --15Claims.

The present invention relates generally to improvements in signalvcurrent transmission systems of the type in whichl signal controlledswitching circuits are provided for blocking, under certain conditions,certain of the signal current channels included thereinand, 'moreparticularly, to improvements in telephone substation circuits havingincorporated therein 'coupled signal current channels for thetransmission of' incoming and outgoing signal currents.

In the usual telephone substation circuit, a hybrid system or antisidetone network is provided for preventing signal currents developed duringoperation of the transmitter from being transmitted to the receiver andfor preventing signal currents -incoming over the line extending to thesubstation from being transmitted to the transmitter. In this type ofarrangement one of the factors which determines the eiliciency oftheside tone suppression is the impedance of the talking circuitestablished by way ofthe two connected subscribers lines. This impedanceis not the same for diierent connections between different subscriberssubstations. Accordingly, the hybrid system of each substation isusually balanced to provide maximum side tone suppression eiliciency foraverage line conditions. In installations wherein amplication of theincoming and outgoing signal currents is required, theconditions ofunbalance introduced 4into the substation circuit by the impedances -ofthe lines over which an established talking This circuit extends maybecome intolerable. is particularly true in substation installationsprovided in an exchange area where the subscribers lines are of widelydierent lengths. As an alternative to the type of system just described,an arrangement may be employed wherein separate signal currenttransmission channels are provided fcrtransmitting outgoing and incomingsignal currents between the associated transmitting and receiving meansand the linerextending to the substation circuit. In such an arrangementthe channels must necessarily be electrically coupled at one end thereofand are usually acoustically coupled at the other end thereof. Due tothe coupling between the channels it is necessary to provide some formof channel blocking means for 'rendering partially or completelyinactive the channel over which signal currents are normally'transmitted to the receiving means during those intervals when thetransmitter is in use, thereby to prevent the locally generated signalcurrents from being reproduced by the receiving means, and

vice versa. Moreover, ii.' ampliiication is used in either or both ofthe two channels and the coupling between the transmitting and receivingmeans is of substantial magnitude, channel blocking is necessary for thefurther purpose of preventing the circuit from oscillating to produceinterference commonly known as singing.

It is an object of the present invention, therefore, to providetelephone substation apparatus of the character described wherein 'theblocking of the signal current receiving channel when the outgoingsignal current transmission channel is in use is accomplished in apositive and reliable manner without the use of movable circuitcontrolling devices and with a minimum of circuit apparatus.

It is a further object oi'y the invention to provide animprovedtransmission system particularhr suited for use in telephonesubstation circuits of the character described wherein the blocking ofone of the signal current channels included thereinv is controlled in asimple, positive and reliable manner in accordance with the direction ofsignal current flow through the system, Y l

In the illustrated embodiment of the invention there is'provided atelephone substation circuit which is adapted to be connected to anassociated I line ,and comprises the usual transmitting and receivingmeans. A transmission system is provided which includes a first channelfor transmitting signal currents from the transmitting means to the lineand a second channel for transmitting incoming signal currents to thereceiving means. This second channel yincludes a repeater whichcomprises an electron discharge tube-having a iirst control electrodeadapted to cause a relatively large change inthe electron stream of thetube in response to a given change in the potential thereonv and asecond control electrodev adapted to cause a relatively small change inthe electron stream of the tube in response to said given potentialchange. The repeater further comprises an input circuit which is coupledto the rstor outgoing signal current transmission channel and isoperative to impress signal voltages of substantially opposite phaseupon the two control electrodes when signal currents traverse thischannel. Apparatus comprising -a voltage transforming circuit includedin the first orv outgoing signal current channel is provided forchanging the relative magnitudes of the voltages impressed upon the twocontrol electrodes in accordance with the direction of signal currentilow through the iirst channel so that only signal currents incoming tothe substation apparatus are transmitted from the first channel throughthe repeater to the second channel. More particularly, the electrondischarge tube is so constructed and arranged that the first and secondcontrol electrodes thereof are adapted to cause changes in the electronstream of the tube in response toa given change in the potentialimpressed thereon which bear a predetermined relatively large ratio toeach other. In order to utilize this relationship, the input circuit tothe tube and, more particularly, the voltage transforming circuitmentioned above is so connected and arranged and the circuit elementsthereof are so proportioned that the ratio of the magnitude of thesignal voltages impressed upon the two control electrodes of the tube issubstantially the inverse of the predetermined ratio mentioned abovewhen signal currents flow from the transmitting means over thefirstchannel to the line andis substantially greater than the inverse ratioJust mentioned when signal currents iiow from the line over the firstchannel to the4 input circuit of the repeater.

The novel features believed to be characteristic of the invention areset 4forth with particularity in the appended claims. The invention,both as to its organization and method of operation, together withfurther objects and advantages thereof will best be understood byreference to the specifi-cation taken in connection with theaccompanying drawing the single iigure of which illustrates a telephonesubstation circuit having incorporated therein the 'features of theinvention as briefly outlined above,

Referring to the drawing, the substation circuit there illustrated isadapted to be connected to a line terminating at the terminals I andcomprises a transmitter or microphone |I and receiving means in the formof a loud speaker I2. The line connected to the terminals III may, forexample, form a part of a conventional automatic or manual telephonesystem in which case it will terminate at its distant end in a linecircuit. the character of which is determined by vthe character of theterminating exchange.

Preferably, the loud speaker I2 is of the wellknown moving coil typehaving embodied therein a voice or signal current coil. I3 which isadapted to be energized by incoming signal currents. For the purpose ofcoupling the transmitter II and-the loud speaker I2 to the lineextending to the terminals Ill, there is provided a transmission systemwhich includes a first channel Il and a second channel I5. The signalmanner illustrated. More particularly, the repeater I9 conprlses anelectron discharge tube 2| of the well-known pentode type whichincludesa' filamentary cathode 22, a first control electrode 23, a screen grid24, a suppressor grid 25 and an anode 26. The output electrodes 22 and26 of the tube 2| are coupled to the input circuit of the power amplier20 through a re sistance capacitance coupling network which comprises aload resistor 21, a voltage dividing resistor 28 and a pair of signalcurrent coupling condensers 29 and 3II.

The electron discharge tube 2| is of the type wherein the suppressorgrid 25 is electrically isolated from the other electrodes of the tubeand is connected to a receptacle plug extending through the .base of thetube so that connections may be made thereto. As thus connected, thisgrid may be utilized as a second control electrode for determining theelectron flow from the cathode 22 to the anode 26. More particularly,this tube is so constructed and arranged that if a biasing potential isimpressed upon the suppressor grid 25 and the potential is changed by agiven amount, only a small change occurs in the electron stream or thenumber of electrons transmitted from the cathode 22 to the anode 26. Inother words, the amplification factor of the tube as measured by controlof the electron stream exercised through changes in the potential of thegrid 25 is relatively small. On the other hand, the amplification factorof the tube is measured by control of the electron stream exercisedthrough changes in the potential of the grid 23 is quite large. 'Ihatis, a given change in the potential impressed upon the grid 23 causes alarge change in the magnitude of the electron stream flowing from thecathode 22 to the anode 26. Tubes, such, for example. `as the type IAGare commercially available having characteristics such that a givenpotential in the potentials impressed upon two of the control electrodesthereof will produce changes in the electron stream having a magnituderatio substantially greater 2| as achieved through control of theelectron transmission channel I4 isV utilized for the purpose oftransmitting signal currents developed through operation of thetransmitter II to the lineconnected to the terminals I0 and is coupledto the channel I5 so that signal currents incoming over the associatedline may be transmitted therethrough to the second channel. Brieflydescribed, the signal transmission chan- ,nel Il comprises a vacuum tubeamplifier I6 for increasing the level of signal currents developedthrough operation of the transmitter Il, a-voltage transforming networkI1 and a coupling transformer I8 which functions to isolate linecurrents from the signal current channels. The incoming signal Acurrenttransmission channel I5 comprises a repeater generally indicated at I3which functions as the driver stage for a power amplifier 20, the outputcircuit of which is coupled to the voice coil I3 of the loud speaker I2in the 75 stream by changing the potentials of the grids 25 and 23,respectively, is utilized to block the second signal current channel I5when signal currents traverse the rst channel Il in a pre` determineddirection. To this end, the control electrode 25 is coupled to thechannel I4 at a point A, intermediate the amplifierv I6 and the voltagetransforming circuit |1, over a path which includes a voltage phasereversing transformer 3| and a potentiometer resistor 32, the latterelement having an adjustable tap 33 connected directly to the grid 25.The grid or control electrode 23, on the other hand, is coupled to thechannel I4 at a point B, intermediate the voltage transforming circuitI1 and the coupling transformer I8, over a coupling path which hasincluded therein a potentiometer resistor 34 having 'an adjustable tap35 directly connected to the grid 23.

The voltage transforming circuit I1 is provided for the purpose ofcreating a diii'erence in the magnitudes of the voltages appearingacross the channel I4 at the points A and B when signal currentstraverse this channel. As will be pointed out'more particularlyhereinafter, one of the requirements of this circuit is that no phaseshift occur therein. In other words, the

signal voltage appearing across the channel I4 at the point A whensignal currents traverse the channel should be in phase coincidence withthe signal voltages appearing across the channel at 'the point B. Forthis reason, the circuit is commagnitude of the signal voltagevappearing across the channel I4 at the point B therealongis less inmagnitude than the voltage across the channel at the point A. In asimilar manner when factor of the grid 23 as compared with that of 'ithe grid 25, greater changes inthe'electron flow occur in response to agiven change in potential upon the grid 23 than in response to the samesignal currents are delivered to the circuit I1 through the transformerI8, the voltage across the channel at the point A differs from and isless than that appearing across the channel at the point B. Preferably,the constants of the circuit I1 are proportioned to provide a drop ofapproximately ten decibels in the signal level between the points A andB in each direction.

For the purpose of supplying positive anode potential to the anode 26 ofthe tube 2| and of providing appropriate biasing potentials for thegrids and 23, there is provided a direct current source 39 which isshunted by a Voltage dividing resistor 40. More particularly, theresistor 40, which is connected across the terminals of the sounce 39,is provided with a grounded midtap 4I through which connection is madeto the grounded cathode 22 of the tube 2| and with two additional taps42 and 43 by way of which negative biasing potentials ofappropriatevalues are respectively impressed upon the control electrodes23 and 25.

Referring now more particularly to the operation of the apparatus, whensound waves impinge upon the diaphragm of the microphone II thegenerated signal currents are amplified by .the ampller I6 and aretransmitted through the voltagetransforming circuit I1 and the couplingtransformer I8 to the line terminating at the terminals I0. During suchsignal current trans- .mission the signal voltage developed across thechannel I4 at the point A is reversed in phase by the transformer '3Iandimpressed across the terminals of the potentiometer resistor 32. The

portion ofV this voltage represented by thedrop across the adjustableportion of the potentiometer resistor 32 is impressed between thecathode 22 and the suppressor grid 25 over a path including theadjustable tap 33. Similarly, the

signal voltage developed across the channel I4 at the point B duringsignal current transmission of the character described is impressedacross the potentiometer resistor 34 and the portion of this voltageappearing across the adjustable portion of the resistor 34 is impressedbeerted by these .two voltages upon the electron change in the potentialimpressed upon the grid 25. With the arrangement described, however, thesignal voltage of largel amplitude is impressed upon the grid 25 whilethe signal voltage of small amplitude is impressed upon the grid 23.More particularly, the resistance values of the elements 36, 31and 38,forming the voltage transforming circuit I1, are so proportioned thatthe ratio of the signal voltages respectively impressed upon the grids25 and 23 when signal currents flow from the ymicrophone II over thechannel I4 to the line connected to the terminals I0 is the inverse ofthe ratio of the changes produced in the electron stream in response toa given change in the potentials impressed upon the grids 25 and 23.Thus, the net control effect exerted on the electron stream flowingrfrom the y cathode 22 to the anode 26 is substantially zero. In orderlfurther toobtain an exact canceling of the control influencesrespectively exerted on the electron stream by the potentials on thegrids 25 and 23, the adjustable taps 33 and 35 may be adjusted asrequired in order to obtain the necessary signal voltage' ratio. Due tothe opposingeffects exerted by the two control grids, no sub- A stantialchange occurs in the output current of the repeater I9 which traversesthe load resistor 21. Accordingly, the signal currents are nottransmitted through the channel I5 to the loud speaker I2 duringoperatlonof the transmitter II, this channel being effectively blockedduring such signal current transmission.

Signal currents incoming to the substation apparatus over the lineconnected to the terminals I0 are transmitted through the couplingtransformer I8 and the voltage transforming circuit I1 to the outputcircuit of the signal amplifier I8. Since this amplifier is inherently aunidirectional transmitting device, the signal currents are preventedfrom flowing. therethroughy tothe y .transmitter II. When, however,signal currents traverse the channel I4,in this direction `the volt-lage appearing acrossthe channel at the point ,B y is substantiallygreater than that appearing' across the channel at the point A due tothe voltage reducing action of the circuit I1. Accordingly, the signalvoltage which is impressed upon the control grid 23 having the highamplification factor is substantially greater than that irn-l currentsare repeated through the repeater I9` and by Way of the resistancecapacitance coupling network included in the output circuit thereof tothe input circuit of the amplifier 20. Following vamplification in theamplifier 20, these signal currentsv are delivered` to the loud speakerI2 for reproduction in the usual manner. From the foregoing explanationit will be apparentv that a substantialdrop in svignavlflevel signalvoltagegl im- Y 4 is realized through the action of the ci'rcuit l1. Inorder to minimize this drop while maintaining the required ratio betweenthe signal voltages impressed uponthe controlelectrodes 23 and 25 anamplifier may be inserted .in the coupling path over which the channelI4 is coupled to the control electrode 25. In such case, the phasereversing transformer 3| vcould be omitted or the input or outputcircuit of the amplifier could include a phase correcting networksuitably designed to maintain the signal voltages impressed upon the twoelectrodes 23 and 25 in phase opposition.

While. there has been described what is at present considered to be thepreferred embodiment of the invention, it will be understood thatvarious modifications may be made therein, and it is contemplated tocover 'in the appended claims all such modifications as fall within thetrue spirit and scope of the invention.

I claim:

1. In a transmission system, a first two-way signal transmissionchannel, a second signal transmission channel, a repeater included insaid second channel,'said repeater including an electron discharge tubehaving a first control electrode adapted to cause a largechange in theelectron stream of the tube in response to a -given change in thepotential impressed thereon and a second control electrode adapted tocause a relatively small change inthe electron stream of the tube inresponse to said given change in the potential impressed thereon, aninput circuit coupled to said rst channel at two different i points andincluding means for impressing signal voltages of substantially oppositephase upon said control electrodes when signal currents traverse saidfirst channel, and means comprising a two-direction transmission networkincluded in said first channel between said points for changing therelative magnitudes of said voltages in accordance with the direction ofsignal s current flow through said first channel so Vthat signalcurrents are transmitted' from said first channel through said secondchannel only whenv signal .currents traverse said first channel in 'a.predetermined direction.

2. In a transmission system, a first -signal transmission channel, asecond signal transmission channel, a repeater included in said secondchannel, said repeater including an electron disl charge tube ,havingfirst and second control electrodes adapted yto cause changes in theelectron stream of the tube in response to a given change in the'potentials impressed thereon having a ratio to each other which isgreater than unity, an vinput circuit for impressing signal voltages ofsubstantially opposite phase upon said control'velectrodes when signalcurrents traverse said first,Y channel, and means for controlling saidvoltages-in accordance with the direction ofsignal .carrent 4flowthrough` said first channel so thatthe .ratio of the magnitudes thereofis substantially-.the inversepof said predetermined ratio whensignalcurrents traverse said first channel in or-iekadirection and issubstantially greater 'than said inversev ratio when signal currentstraverse saidflrst Achannel in the opposite direction.A c

3.In-a transmission system, a first signal transmission channel, asecond signal transmission channel, a repeater included in said secondchannel, said repeater including an electron discharge tube having firstand second control electrodesfadapted to cause changes in the electron`asesinos electrodes when signal currents traverse said I first channel,and means comprising a voltage reducing circuit included in said firstchannel for controlling said voltages in accordance with the directionof signal current flow through said first'channel so that the ratio ofthe magnitudes thereof is substantially the inverse of saidpredeterminedratio when signal currents traverse said first channel in one directionand is substantially greater than said inverse ratio when signalcurrents traverse said first channel in the opposite direction.

4. In a transmission system, a first two-way signal transmission channelincluding a voltage transforming circuit, a. second signal transmissionchannel, a repeater including an electron discharge tube having a firstcontrol electrode adapted to cause a large change in the electron streamof the tube in response to a given change in the potential impressedthereon and a second control electrode adapted to cause a relativelysmall change in the electron stream of said tube in response to saidgiven change in the potential impressed thereon, a path for couplingsaid rst control electrode to said first channel at a point on one sideof said `voltage transforming circuit, a path including a voltage phasereversing device for vcoupling said second control electrode to saidfirst channel at a point on the other side of said voltage transformingcircuit, and means including said voltage transforming circuit forcontrolling the relative magnitudes of the signal voltages impressed ,onlsaid control electrodes over said paths so that signal currents aretransmitted from said first channel to said second channel only whenthey traverse said first channel in a predetermined direction.

5. In a transmission system, a first signal transmission channelincluding a voltage transforming circuit, a second signal transmissionchannel, a repeater including an electron discharge tube having firstandsecond electrodes adapted t0 causechanges in the electron stream ofthe tube in response to a given change in the potentials impressedthereon having ratio to each other which is greater than unity, a pathfor coupling said first electrode to said first channel at a point onone side of said voltage transforming circuit; and a path including avoltage phase reversing device for coupling said second controlelectrode to said first channel at a point on the other side of saidvoltage transforming circuit, the impedances of the circuit elementsincluded in said-paths and said voltage transforming circuit being soproportioned that the ratio of the magnitudes of the signalvoltagesimpressed on said control electrodes over said paths issubstantially the inverse of said predetermined ratio when signalcurrents traverse said first channel in one direction and issubstantially greater nel, a repeater including an electron dischargetube having a rstcontrolelectrode adapted to cause a large change in theelectron stream of the tube in response to a given change in thepotential impressed thereon and a second control electrode adapted tocause a relatively small change in the electron stream'of said tube inresponse to said given change in the potential impressed thereon, a pathfor coupling said first control electrode to said first channel at apoint on one side of said voltage transforming circuit, a path includinga voltage phase reversing device for coupling said second controlelectrode to said i'lrst channel at a point on the'other side of saidvoltage transforming circuit, and means including said voltagetransforming circuit for controlling the relative magnitudes of thesignal voltages impressed on said control electrodes over said paths sothat signal currents are transmitted from said rst channel to saidsecond channel only when they traverse said first channel in apredetermined direction. y

'7. Telephone substation apparatus adapted to be connected to a line andcomprising transmitting and receiving means, a first two-way channel fortransmitting signal currents from said transmitting means to said lineand for transmitting incoming signal currents to said receiving means, asecond channel coupling said receiving means to said rst channel, arepeater included in said second channel and comprising an electrondischarge tube having a rst control electrode adapted to cause a largechange in the electron stream of the tube in response to a given changein the potential impressed thereon and a second control electrodeadapted to cause a relativelysmall changein the electron stream of thetube in response to said given change in the potential impressedthereon, an input circuit coupled to said rst channel at two differentpoints and including means for impressing signal voltages ofsubstantially opposite phase upon said control electrodes when signalcurrents traverse said first channel, and means comprising atwo-direction transmission network included in s'aid first channelbetween said points for controlling the relative magnitudes of saidvoltages in accordance with the direction of signal current ow throughsaid rst channel so that only signal currents incoming to said apparatusare transmitted from said rst channel said second channel.

' 8. Telephone substation apparatus adapted to be connected to a lineand comprising transmitting and receivingmeansj a first channel fortransmitting signal currents from said transmitting means to said line,a second channel for transmitting incoming signal currents to saidreceiving means, a repeater included in said second channel andcomprising an electron discharge tube having first and second electrodesadapted to cause changes in the electron stream of the tube in responseto a given change in the potentials impressed thereon having ratio toeach other which is greater than unity, an input circuit coupled to saidfirst channel and operative to impress signal voltages of substantiallyopposite phase upon said control electrodes when signal currentstraverse said first channel, and means for controlling said voltages inaccordance with the direction of signalcurrent flow through said channelso that the ratio of the magnitudes thereof is substantially the inverseof said predetermined, ratio when signal currents-now from saidtransmitting means through said first channel to said line and issubstantially greater than y v said inverse ratio when signal currentsflow fromv said line over said first channel to said input circuit.

9. Telephone substation apparatus adapted to be connected to a line andcomprising transmitting and receiving means, a rst channel fortransmitting signal currents from said transmitting means to said line,a second channel for transmitting incoming signal currents to saidreceiving means, a repeater included in said second channel andcomprising an electron discharge tube having first and secondelectrodes-adapted to cause changes in the electron stream of the tubein response to a given change in the potentials impressed thereon havingratio to each other which is greater than unity, an input circuitcoupled to said first channel for impressingv signal voltages ofsubstantially opposite phase upon said control electrodes when signalcurrentsk traverse said. first channel, and means comprising a voltagetransforming circuit included in said rst channel for controlling saidvoltages in accordance withthe direction of signal current flow circuitincluded in said rst channel, a second channel for transmitting incomingsignal cur- -rents from said line to said receiving means, a

repeater included in said second channel and comprising a first controlelectrode adapted to cause a large change in the electron stream of thetube in response to a given change in the potential impressed thereonand a second control electrode adapted to cause a relatively smallchange in. the electron stream of the tube in response to said givenchange in the potential impressed thereon, a path for couplingsaid firstcontrol'electrode to said first channel at a point on one side of saidvoltage transforming circuit,

a path including a voltage phase reversing device for coupling saidsecond control electrode 'to said first channel at apoint on the otherside of said voltage transforming circuit, and means" including saidvoltage transforming circuit for controlling the relative magnitudes ofthe signal voltages impressed on said control electrodes over said pathsso that only signal currents incoming to said apparatus are transmittedfrom said first channel through said second channel.

11. In a transmission system, a bi-directional signal currenttransmission channel including a voltage transforming network operativeto pass signal currents in either direction, a second signal currentchannel coupled to said bi-directional Y channel at points on eitherside of said network, and means controlled in accordance with thedifference between the signal voltages across said channel at points oneither side of saidnetwork for preventing signal currents transmittedover said bi-directional channel in one direction from traversing saidsecond channel.

l2. In a transmission system, a bi-directional signal currenttransmission channel including a voltage transforming network operativeto pass signal currents in either direction, a second signal 'currentchannel including an electron discharge device having a pair of controlelectrodes coupled to said bi-directional channel on either Y side ofsaid network, and means including said the opposite direction, wherebysignal currents .are transmitted from said bi-directional channelthrough said second channel only when they traverse said bi-directionalchannel in said other direction.

13. In a transmission system, a bi-directional signal currenttransmission channel, a coupling network including anA output circuitand means for transmitting signal currents from said chan-` nel to saidoutput circuit only `when they traverse said channel in a predetermineddirection. and a second signal current transmission channel electricallycoupled to said oi-directional chanlnel only through said network,whereby signal currents are only transmitted from said bidirectionalchannel through said network to said 30 second channel when theytraverse said bidirectional channel in said predetermined direction.

14. In a transmission system, a lai-directional signal currenttransmission channel including a voltage transforming network operativeto pass signal currents in either direction. a coupling networkcoupledto said channel at points on either side of said voltagetransforming network and includingl an output circuit and meanscontrolled in accordance with the difference in the signal voltagesacross said channel at said points for transmitting signal currents fromsaid channel to said output circuit only when the signal currentstraverse said channel in a predetermined direction, and a second signalcurrent transmission channel electrically coupled to said bi-directionalchannel only through said coupling network, whereby signal currents areonly transmitted from said bi-directional channel through said networkto said second channel when they traverse said bi-directional channel insaid predetermined direction.

15. In a transmission system, a bi-directional signal currenttransmission channel including a voltage transforming network operativeto pass signal currents in either direction, said network consisting ofinterconnected resistance elements exclusively, a coupling networkcoupled to said channel at points on either side of said voltagetransforming network and including an output circuit and meanscontrolled in accordance with the difference in the signal voltagesacross' said channel at said points for transmitting signal currentsfrom said channel to said output circuit only when the signal currentstraverse said channel in a predetermined direction, and a second signalcurrent transmission channel electrically coupled to said.bi-directional channel only through said coupling network, whereby`signal currents are only transmitted from said bi-directional channelthrough said network to said second channel when they traverse saidbi-directional channel in said predetermined direction. f

ROSWELL H. HERRICK.

